In knackers' yards, animal carcasses are cut up into chunks of flesh and bone that are about 30 mm a side. The chunks have an average moisture content of about 60%, and they are fed into a cooker where they loose a large part of their moisture, leaving the cooker as a kind of flour with about 10% moisture content. The cooker comprises an oven which is heated by steam at 150.degree. C. The heating steam is produced in a boiler which burns fuel oil. The moisture given off from the animal matter fed into the cooker leaves the cooker in the form of greasy steam which is condensed in a condenser using cooling water which is itself cooled, after passing through the condenser, by means of an air cooler. The water condensed in the condenser is extremely dirty and is disposed of via the sewers. In an installation of this type, the heat in the greasy steam is simply lost to the atmosphere in the air leaving the air cooler. A great deal of heat is thus lost condensing the greasy steam.
Preferred embodiments of the present invention recover a considerable portion of the latent heat in the greasy steam given off by heated animal matter.